1. Field of the Invention
This invention relates to a high-frequency power amplifier, and more particularly to a miniature high-frequency power amplifier module, for amplifying high-frequency power in, for example, a mobile phone or a portable terminal.
2. Description of the Related Art
With the demands for the miniaturization of terminals and longer calls, a high-frequency power amplifier at the last stage of the signal transmitting section of a portable terminal is required to be smaller and have higher efficiency. In recent years, the demand for miniaturization has been particularly getting stronger. To meet the demand, power amplifiers have been modularized in such a manner that transistors and peripheral parts constituting an amplifier are squeezed into a small package, with the input and output impedances matched to 50 Ω. The size of the module is 4 to 5 mm square.
One of the performance characteristics the module is required to have is an easy-to-use feature. Specifically, the module is required to have its input and output matched with specific impedances and be excellent in surge resistance, temperature characteristic, and stability to voltage fluctuations. Furthermore, the module is needed to be stable to fluctuations in an external circuit. Specifically, a power amplifier is provided near the exterior, via a passive component, such as a switch or a duplexer. For this reason, the amplifier is required not to malfunction, such as break down or oscillate, when there are expected fluctuations in the external situation, such as load fluctuations, for example, the damage to the antenna or the deformation of the antenna.
One typical example of such a malfunction is f0/2 oscillation. This occurs at half of the transmission frequency f0, when the load is changed greatly in a state where a transmission signal of a frequency of f0 is being inputted. Several causes of the oscillation are considered. Hereinafter, one of the causes will be explained.
FIG. 1 schematically shows a circuit of a two-stage amplifier constituting a power amplifier module. The power amplifier module comprises a GaAs chip 11 acting as an amplifier section, an input matching circuit 12, an output matching circuit 13, current supply lines (power supply lines) ML1, ML2 having, for example, inductance components L1, L2, and capacitors C1, C2 composed of chip parts. The GaAs chip 11, which is composed of a heterojunction bipolar transistors (HBT), comprises a transistor Q1 which constitutes a first-stage amplifier, a transistor Q2 which constitutes a second-stage amplifier an MIM (Metal Insulator Metal) capacitor C3, and a bias circuit 14.
The input matching circuit 12 is connected between an input pin Pin and the base of the transistor Q1. The output matching circuit 13 is connected between the collector of the transistor Q2 and an output pin Pout. The line ML 1 having the inductance component L1 is connected between a power pin Vcc1 and the collector of the transistor Q1. The line ML 2 having the inductance component L2 is connected between a power pin Vcc2 and the collector of the transistor Q2. The capacitor C1 is connected between the power pin Vcc1 and the ground and the capacitor C2 is connected between the power pin Vcc2 and the ground. A load Zout is connected between the output pin Pout and the ground.
The load impedance of the transistor Q2 is originally designed to operate in a linear or slightly saturated region. However, for example, when an antennal (not shown) connected to the power amplifier module has broken, the value of the load Zout fluctuates seriously. The fluctuation in the load Zout causes the load impedance of the transistor to take a different value from the designed value. Under this condition, if a signal is outputted at the output pin Pout, reflected waves will be generated at the load Zout, which can cause a large current to flow or a great voltage amplitude to occur in the transistor Q2. As a result, the transistor Q2 operates nonlinearly, resulting in a distorted waveform. At this time, the transistors Q2 operate in an equivalent manner to a so-called mixer and therefore have a frequency conversion gain.
As shown in FIG. 1, when there is a circuit constituting a feedback loop LP1 in the power amplifier module, the conversion gain the transistors Q1, Q2 have can permit the loop gain to exceed “1” for a signal of a frequency of f0/2. In this state, even if there is a little noise signal in the f0/2 frequency signal, the signal level increases gradually and oscillation occurs in the end. This is called f0/2 oscillation.
In the case of a large module not miniaturized, the distance between the lines ML1, ML2 having the inductance components L1, L2 shown in FIG. 1 can be made sufficiently large. As a result, the electromagnetic coupling between the inductance components L1, L2 forming the loop LP1 does not exist, or only a very weak electromagnetic coupling exists between them. Thus, in the case of the large module, its loop gain does not exceed “1” and therefore f0/2 oscillation will not take place.
However, modules are being miniaturized as described above. Consequently, signal lines are arranged in a narrow region of the module with very high density. Thus, the electromagnetic coupling between the lines becomes stronger, which permits the loop gain to exceed “1” and makes it easier for f0/2 oscillation to take place.
The power supply line ML2 for the transistor Q2 is particularly designed to have such a high impedance that it can be ignored with respect to the output matching circuit 13. To obtain a high impedance, the line ML2 needs a very long line length. A typical example of the line ML2 is a λ/4 line having a length of λ/4. Here, λ is the wavelength of the signal of the frequency f0. It is difficult to form the long line at the surface of the module in which chip components are arranged. For this reason, the module substrate is designed to have a multilayer structure and long lines are realized using a plurality of wiring layers.
On the other hand, the power supply line ML1 for the transistor Q1 also serves as a matching circuit in the circuit shown in FIG. 1. Thus, the power supply line needs to be long to some extent. Since a long line needed when especially frequency is low, in a small module, it is difficult to form the line only at the surface layer of the substrate. For this reason, the line is formed in a layer within the substrate.
As described above, in a conventional multilayer module substrate, the power supply lines ML1, ML2 for the transistors Q1, Q2 are laid complicatedly in the same layer. Therefore, the electromagnetic coupling between the lines ML1, ML2 cannot be avoided. As a result, the loop LP1 shown in FIG. 1 is formed, which causes a problem: f0/2 oscillation takes place when the load fluctuates as described above.
Furthermore, there is a possibility that a loop LP2 will occur in addition to the loop LP1 shown in FIG. 1. The loop LP2 is formed by the electromagnetic coupling between the line ML3 constituting the input matching circuit 12 and the power supply line ML1 for the transistor Q1. The line constituting the input matching circuit 12 also needs to be long to some extent. Thus, these lines may also be formed in an inner layer. As a result, in this case, too, f0/2 oscillation takes place.
As described above, a small module using a conventional multilayer-structure substrate has the following problem: when the transistors operate nonlinearly due to disturbance, such as a fluctuation in the load, the loop gain exceeds “1,” which induces f0/2 oscillation.
One patent reference related to this technique is Jpn. Pat. Appln. KOKAI Publication No. 2000-357771. In this reference, a power supply line 25 for supplying direct-current power to an active element is provided in a layer between a ground layer 39 and a ground layer 40. However, the technique disclosed in this patent reference is for preventing alternating-current noise components caused by a high-frequency circuit from entering the direct-current power supply. Accordingly, the reference has implied neither a decrease in the loop gain nor prevention of f0/2 oscillation.
Accordingly, a high-frequency power amplifier module capable of preventing f0/2 oscillation is demanded.